This invention relates to a method for transmitting a charging signal used for computing the cost of a call, and more particularly to a signal transmitting system where a telephone exchange and public telephone systems are connected via a radio link and the charging signal sent from the exchange is transmitted via radio equipment.
In a telephone service system connecting the exchange and the public telephone systems with the radio link, the charging signal generated by the exchange must be sent to the public telephone systems via the radio link. As this charging signal, a 50 Hz or 16 Hz signal or a polarity-inverting signal are used and charges are communicated to users by transmitting such a signal at particular time intervals.
This charging signal is transmitted during a converation along with the voice signal and, therefore, it is essential to prevent interference with the voice signal caused by such a charging signal.
Moreover, because this signal is used for computing user charges, this charging signal must be transmitted with high reliability and must be provided in a manner which prevents an erroneous action by a charge processing circuit provided in the public telephone system.
When transmitting a voice signal, etc. utilizing radio equipment, the amplitude of an input signal is limited in a circuit preceding a modulator stage in order to operate the modulator of the radio equipment with sufficient dynamic range and to transmit the signal within a predetermined frequency band. Particularly, it is important to prevent the charging signal from disappearing due to this amplitude limitation in the stage preceding such a modulator, when transmitting the charging signal by superimposing it on the voice signal using a frequency signal outside the frequency band used for transmitting the voice signal which is used as the carrier for the charging signal.